Exacerbation frequency generally increases with declining lung function. However, some patients with COPD consistently experience a higher rate of exacerbations than others despite similar severity of COPD. This has led researchers to postulate the existence of a distinct subgroup of "frequent exacerbators" . Recent work has also brought attention to a subset of patients who experience remarkably few exacerbations despite significantly impaired lung function. Careful characterization of both of these extreme subgroups of COPD may offer additional insights into why certain patients are prone to frequent exacerbations while others remain relatively protected.